Illuminator for dark field inspection

ABSTRACT

Light from a single source is divided among several illumination arms, each of which directs light via a multimode fiber bundle from the source to the wafer location. The arms are arranged circumferentially around a common illumination region, so that the region is illuminated from several directions. For each arm, light exiting the fiber bundle enters a turning prism, reflects off the hypotenuse of the prism, and is diverged in one dimension by a negative cylindrical surface on the exiting face of the prism. The beam then reflects off an anamorphic mirror and propagates to the illumination region on the wafer. The beam has an asymmetric footprint, so that it illuminates a nearly circular region of the wafer when viewed at normal incidence. The fiber bundle is at the front focal plane in the meridional dimension. The illumination region is at the rear focal plane in both dimensions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/587,206, filed Jul. 12, 2004, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to inspection of materials, and moreparticular to illuminators for dark-field inspection of material,inspection systems for dark-field inspection of material, and methodsfor illuminating a target for dark-field inspection.

2. Description of the Related Art

As more manufacturing processes use wafer-based technology, and utilizeincreasingly smaller features on these wafers, it becomes increasinglyimportant to inspect the wafers for defects at various stages throughoutthe manufacturing process. Wafer inspection systems have evolved to keeppace with these demanding requirements.

In a typical wafer inspection system, a wafer is attached to a movablestage that translates the wafer underneath an inspection camera. Thecamera acquires an image of a portion of the wafer and processes itusing known image processing techniques. Any defects in the field ofview are either marked on the wafer by the inspection system, or notedin a data file, so that the defect may be corrected or avoided duringsubsequent manufacturing steps. The translation stage then moves thewafer to the next location, and the process is repeated until the entireusable area of the wafer is inspected. This inspection process improvesyields and efficiency by identifying or removing defective material asearly as possible in the manufacturing process.

One class of inspection system especially suited for the inspection ofbare, flat, or featureless wafers is known as “dark-field”. In adark-field inspection system, the illumination takes place at angles ofincidence that are not specularly reflected into the imaging optics. Ifthere are no three-dimensional features or defects in the field of viewon the wafer, the illumination beam undergoes a specular reflection, andis not captured by the imaging optics. In other words, in a dark-fieldsystem, a defect-free wafer appears dark to the imaging camera. A smallparticle, say 5 microns in size, reflects or scatters some light invarious directions, including a fraction that enters the imaging camera.Therefore, a small particle or defect shows up as a bright spot in thefield of view of a dark-field system. A dark field system may also beused for wafers with features on them.

The illumination for a dark-field system greatly affects the overallperformance of the inspection system. For instance, a “ring-lamp”device, similar to those used to illuminate dark-field microscopes, doesnot work very well for the dark-field wafer inspection systems. Theytypically have neither uniform illumination over the field nor a uniformangular spectrum over the field. As a result, the sensitivity of theinspection system can vary over the field of view, which is highlyundesirable. This variation in sensitivity can require additionalprocessing time for each image, thereby slowing down the system andreducing efficiency.

BRIEF SUMMARY OF THE INVENTION

Advantageously, the present invention provides for a dark-fieldillumination system that is compact, or illuminates a large illuminationregion on the target, or provides uniform illumination and a uniformangular spectrum across an illumination region on the target, or has alow range of incident angles and a broad circumferential azimuth for anillumination region on the target, or is bright throughout anillumination region on the target. The various embodiments of theinvention respectively achieve one or more of these advantages.

One embodiment of the present invention is an illuminator for dark fieldinspection of a surface of a target, the illuminator comprising aplurality of illumination arms disposed about an illumination region,and each of the arms comprising: an extended, substantially uniformsource of diverging light; a turning element having a first faceoptically coupled to the light source, and a second face havingcylindrical optical power in a meridional direction; and an anamorphicelement having a first optical power in an azimuthal direction and asecond optical power in the meridional direction, the first and secondoptical powers being unequal, and the anamorphic element being opticallycoupled to the second face of the turning element.

Another embodiment of the present invention is an illuminator for darkfield inspection of a surface of a target, the illuminator comprising aplurality of illumination arms disposed about a substantially circularillumination region substantially at the target surface, and each of thearms comprising: an extended, substantially uniform source of diverginglight; a coupling element having an incident face optically coupled tothe light source, and an exiting face with negative cylindrical opticalpower in an meridional direction; and an anamorphic element having afirst optical power in an azimuthal direction and a second optical powerin the meridional direction, the first and second optical powers beingunequal, and the anamorphic element being optically coupled to theexiting face of the coupling element; wherein each of the arms has anazimuthal front focal plane, an azimuthal rear focal plane, a meridionalfront focal plane, and a meridional rear focal plane; and wherein foreach of the arms: the azimuthal front focal plane and the meridionalfront focal plane are not coincident; the light source is locatedsubstantially at the meridional front focal plane; the meridional rearfocal plane and the azimuthal rear focal plane are substantiallycoincident; the illumination region has a center located substantiallyat the meridional rear focal plane and at the azimuthal rear focalplane; and the light source, turning element, and anamorphic element arearranged to illuminate the illumination region at a non-zero angle, orrange of angles, of incidence relative to a normal of the illuminationregion to achieve specular reflection from the target surface underdefect-free conditions.

Another embodiment of the present invention is an illuminator for darkfield inspection of a surface of a target, the illuminator comprising aplurality of illumination arms disposed about an illumination regionsubstantially at the target surface, and each of the arms comprising: anextended, substantially uniform source of diverging light; and anassembly of optical elements for producing from the diverging light anilluminating beam directed to the illumination region, the illuminatingbeam having a nominal incident angle, an incident angle range, and anazimuthal angle range; wherein both the nominal incident angle andincident angle range are essentially invariant throughout theillumination region; wherein the azimuthal angle range is essentiallyinvariant throughout the illumination region; and wherein lightintensity throughout the illumination region is essentially invariant.

Another embodiment of the present invention is an illuminator for darkfield inspection of a surface of a target, the illuminator comprising aplurality of illumination arms disposed about a substantially circularillumination region substantially at the target surface, and each of thearms comprising: means for establishing an azimuthal rear focal plane;means for establishing a meridional rear focal plane, the meridionalrear focal plane and the azimuthal rear focal plane being substantiallycoincident; means for establishing an azimuthal front focal plane; meansfor establishing a meridional front focal plane separated from theazimuthal front focal plane; means for locating a center of theillumination region substantially at the meridional rear focal plane andat the azimuthal rear focal plane; means for introducing light havingextended, substantially uniform, and diverging characteristicssubstantially at the meridional front focal plane; and means for forminga beam from the light introduced at the meridional front focal plane,the beam being incident on the illumination region at a non-zero angleof incidence relative to a normal of the illumination region to achievespecular reflection from the target surface under defect-freeconditions.

Another embodiment of the present invention is a method for illuminatingan illumination region on a surface of a target from a plurality ofdifferent directions with respective optical systems to perform a darkfield inspection of the target surface, comprising for each of theoptical systems: establishing an azimuthal rear focal plane;establishing a meridional rear focal plane, the meridional rear focalplane and the azimuthal rear focal plane being substantially coincident;establishing an azimuthal front focal plane; establishing a meridionalfront focal plane separated from the azimuthal front focal plane;establishing the illumination region as a substantially circular regionwith a center substantially at the meridional rear focal plane and atthe azimuthal rear focal plane; introducing light having extended,substantially uniform, and diverging characteristics substantially atthe meridional front focal plane; and forming a beam from the lightintroduced at the meridional front focal plane, the beam being incidenton the illumination region at a non-zero angle of incidence relative toa normal of the illumination region to achieve specular reflection fromthe target surface under defect-free conditions.

Another embodiment of the present invention is A wafer inspection systemcomprising: a fiber optic dark-field illuminator comprising: a source ofspectrally filtered, spatially uniform, extended and diverging light;and a plurality of illumination arms optically coupled to the lightsource and circumferentially disposed about a substantially circularillumination region; and a camera disposed at normal or near-normalincidence with respect to the illumination region, the illuminationregion being within a field of view of the camera, and the cameracomprising a collection lens having a numerical aperture that defines amaximum collection angle of light for the collection lens; wherein eachof the illumination arms comprises: a turning element having an incidentface optically coupled to the light source, and an exiting face withnegative cylindrical optical power in a meridional direction; and ananamorphic mirror having a first optical power in an azimuthal directionand a second optical power in the meridional direction, the first andsecond optical powers being unequal, and the anamorphic mirror beingoptically coupled to the exiting face of the turning prism; wherein eachof the illumination arms has an azimuthal front focal plane, anazimuthal rear focal plane, a meridional front focal plane, and ameridional rear focal plane; and wherein for each of the arms: theazimuthal front focal plane and the meridional front focal plane are notcoincident; the light source coupling is located substantially at themeridional front focal plane; the meridional rear focal plane and theazimuthal rear focal plane are substantially coincident; theillumination region has a center located substantially at the meridionalrear focal plane and at the azimuthal rear focal plane; and the lightsource coupling, turning element, and anamorphic mirror are arranged toilluminate the illumination region at an angle of incidence relative toa normal of the illumination region that is greater than the maximumcollection angle.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective drawing of a wafer inspection system.

FIG. 2 is a plan drawing of an illumination system.

FIG. 3 is a top view drawing of the illumination arms, the illuminationregion and the wafer.

FIG. 4 is a plan drawing of a turning prism with a cylindrical elementand an anamorphic reflector.

FIG. 5 is a plan drawing of a turning prism with a cylindrical element,an anamorphic reflector, the illumination region and the wafer.

FIG. 6 is a perspective drawing of an illumination arm, the illuminationregion and the wafer.

FIG. 7 is a side-view drawing of an illumination arm, the illuminationregion and the wafer.

FIG. 8 is a perspective drawing of a cone of light received by aparticular location in the illumination region of the wafer under test.

FIG. 9 is a schematic drawing of a paraxial representation of theoptical system of an illumination arm.

FIG. 10 is a spreadsheet showing a numerical paraxial raytrace of thesystem of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to illuminators for dark-fieldinspection systems, which are commonly used in systems that visuallyinspect wafers for defects during a particular manufacturing process. Anexample of a wafer inspection system is shown in FIG. 1.

A typical wafer inspection system 10 is used in one environment toinspect whole wafers before die have been fabricated on them, but mayalso be used to inspect patterned whole wafers, die diced from patternedwafers, sawn wafers, broken wafers, wafers of any kind on film frames,die in gel paks, die in waffle paks, MCMs, JEDEC trays, Auer boats, andother wafer and die configurations, whether or not packaged. Hereafter,all of these uses shall be referred to generally as inspection ofwafers. System 10 includes a wafer test plate 12, an actuator 14 thatmoves the wafer to the test plate 12, a wafer alignment device 16 foraligning each and every wafer at the same x, y, and angular location orx, y, z, and angular location, a focusing mechanism 18, a camera 20 orother visual inspection device for visual inputting of good wafersduring training and for visual inspection of other unknown qualitywafers during inspection, a user console 22 for inputting parameters andother constraints or information such as sensitivity parameters,geometries, die size, die shape, die pitch, number of rows, number ofcolumns, etc., a display 24 for displaying the view being seen by thecamera presently or at any previously saved period, a computer system 26or other computer-like device having processing and memory capabilitiesfor saving the inputted good die, developing a model therefrom, andcomparing or analyzing other die in comparison to the model, a frame 30,a hood 32, a control panel 34, a system parameters display 36, objective38, and a fiber optic dark field illuminator 40.

Generally, a dark field illuminator may have one or more illuminationarms, each having a light source. In one embodiment, the dark fieldilluminator has a single light generating element, the output of whichmay be spectrally filtered, collected, made spatially uniform, and/ordivided among one or more illumination arms. Alternatively, eachillumination arm may be provided with its own light generating element,the output of which may be spectrally filtered, collected, and madespatially uniform. In those embodiments where multiple illumination armsare provided as illustrated in FIG. 2, each illumination arm emits lightfrom a particular position around the circumference of an illuminationregion to illuminate the illumination region, with enough spatialclearance for a turret of lenses for an inspection camera and variousother mechanical elements. Note that while five illumination arms areillustrated, it is to be understood that more or fewer arms may beprovided. Generally, the illumination arms may be located in any desiredmanner with respect to the illumination region, provided that theillumination is delivered at a fairly high angle of incidence relativeto the surface normal from each illumination arm to illuminate theillumination region so that a specular reflection from the wafer is notnormally collected by the imaging optics. In this manner, a no-defectcondition appears as darkness in the visual field, hence the name “darkfield”. The numerical condition for dark field illumination is asfollows: if the collection (or imaging) optics have a characteristicnumerical aperture NA, which defines the maximum angle at which lightcan enter the collection optics, then the dark field illumination systemshould supply light at an effective numerical aperture larger than NA,so that it does not enter the collection optics for a specularreflection.

Note that in general, a dark field system need not be constrained to thecondition in which the collection optics are oriented at near-normalincidence and the illumination optics are at a higher angle ofincidence, located “outside” the field of the collection optics. Forinstance, the locations of the illumination and collection systems maybe reversed, with the illumination system surrounded by the collectionsystem. Or, particular angles or orientation may be physically blockedin the pupil of the lens. For the purposes of this document, thecollection optics are oriented at near-normal incidence, and theillumination optics are oriented at near-grazing incidence, well outsidethe numerical aperture of the collection optics.

For an optimal signal-to-noise ratio, it is desirable to maximize thegap between the maximum collection angle and the minimum illuminationangle. For instance, if the collection optics have a numerical apertureof 0.8, then they collect all the light in the field of view having apropagation angle less than 53°. If the illumination system supplieslight at an incident angle of 81°±2.5°, then the minimum illuminationangle is 78.5°, and the angular separation between the illumination andthe collection bands is 25.5°. This relatively large angle improves thesignal-to-noise ratio for detecting small objects or particles in thefield of view.

Note that for the purposes of this document, the illumination region isdefined as the area to be illuminated when the system is operational. Itwill be understood that even when the system is not operational or thelight source is between pulses, the illumination region still exists,even though it is not receiving light from the source.

A schematic drawing of the illuminator 40 is shown in a disassembledstate in FIG. 2. Light is produced for the illumination system by alight source, which is one embodiment, a strobe lamp 42. The strobe lamp42 is activated to illuminate the illumination region for a short periodof time each time a picture is taken by the camera (FIG. 1, element 20).Use of such a strobe lamp 42 is effective at reducing the blur thatmight occur if a continuously operating light source were used with awafer that was subject to movements from the translation stage. In oneembodiment, the lamp contains a fairly broad spectrum of wavelengths,but may be adapted to output a more narrow spectrum of wavelengths whereso desired.

Upon leaving the strobe lamp 42, the beam passes through awavelength-sensitive filter 41, which blocks wavelengths in the range ofabout 280 nm to 400 nm, and passes wavelengths longer than about 400 nm.This wavelength range is merely exemplary, and any suitable filter maybe used. Alternatively, the filter may be omitted.

The wavelength-filtered beam then enters a condenser lens 45, which maybe spherical or aspheric, and may be coated with an antireflection thinfilm coating. The condenser lens 45 directs the beam into a mixing bar43, which is effectively a long, skinny, transparent parallelepiped.Light enters one of the longitudinal ends of the mixing bar 43 andbounces through total internal reflection until it leaves the mixing bar43 at the other longitudinal end. The output from the mixing bar 43 isessentially uniform in intensity, and also essentially uniform inangular spectrum within a particular numerical aperture.

The combination of the condenser lens 45 and the mixing bar 43 may bereferred to as the head 44.

The output from the mixing bar 43 is coupled into multiple fiber bundles48, where each fiber bundle 48 directs a beam with its own set of opticsonto the illumination region. In some embodiments, the illuminationregion defines all or a portion of wafer of the type used in themanufacture of semiconductor and micro-electromechanical devices. Thefibers used in the bundle are in some embodiments multi-mode fibers thatefficiently couple in light within a particular numerical aperture. Toget efficient coupling from the mixing bar 43 to the fiber bundles 48,the numerical aperture of the fiber bundles 48 may be matched to thenumerical aperture of the beam exiting the mixing bar 43. The fibers arearranged in a bundle, which may be round or some other convenient shape,and may have different shapes or arrangements at each end.

The fiber bundles 48 direct the light toward a wafer or other objectwithin the illumination region, and are arranged to encircle the cameraand direct light toward the illumination region at a large angle ofincidence. The illumination optics are designed to allow for a largeclearance so that the camera and other mechanical elements have enoughroom to operate or be adjusted.

Each fiber bundle 48 is coupled to a right-angle prism 52, which turns abeam exiting the bundle 90° after a reflection off the prism hypotenuse.The exiting face of the prism has a negative cylindrical surface, whichdiverges the beam in the x-direction and leaves the beam largelyunchanged in the y-direction. Such a prism is straightforward tomanufacture, and is typically made first as a 45/45/90 prism, upon whichthe cylindrical surface is then ground and polished. Alternatively, acylindrical lens may be attached to the exiting prism face, so that thecylindrical surface does not have to be manufactured directly on theprism. The 90° angle as shown is preferred, so that the entire opticalpath can fit inside a fairly small volume in the machine. Although a 90°turning prism is shown, any other suitable turning angle may be used.Likewise, any other suitable prism may be used, with any number ofinternal bounces, including no bounces. Alternatively, the turning prismmay be replaced by an element that does not turn the beam at all, butmerely has a cylindrical optical power. For instance, a cylindrical lensmay be attached to the fiber bundle without a prism at all.

The beam then reflects off an anamorphic mirror 54, which has differentpowers in the x- and y-directions. Additionally, the anamorphic mirror54 may have aspheric terms along one or both of the directions, tocorrect for aberrations and improve the wavefront quality of the beam.The anamorphic mirror may be used at non-normal incidence, so that theincident and exiting beams do not interfere with or block each other.Alternatively, an anamorphic lens may be used instead of an anamorphicmirror, in which case the anamorphic lens may be used on-axis and mayhave aspheric terms along one or both of the directions to correct foraberrations.

The combination of the prism 52 and the mirror 54 may be considered tobe part of an illumination arm 46 or an arm.

After reflecting off the mirror 54, the beam strikes the illuminationregion at a fairly high angle of incidence. The shape of the beamleaving the mirror 54 is elliptical, so that the illuminatedillumination region is essentially round when viewed from a camera at ornear normal incidence.

FIG. 3 shows the illumination region 50 of a wafer 56 from the point ofview of the camera, at near-normal incidence. The illumination arms46A-E are arranged around the circumference of the illumination region50, which essentially corresponds to the illumination region. Note thatbecause the illumination region 50 is smaller than the wafer 56; inorder to inspect the entire wafer surface, the wafer 56 may be scannedwith respect to the illumination region 50. The wafer may be placed on acomputer-controlled translation stage that can automatically scan theentire wafer 56, while the inspection system acquires, stores andprocess the acquired images. Alternatively, the wafer may remain fixedand the illumination and acquisition optics translated.

FIG. 4 and FIG. 5 show close-up views of the optics of the illuminationarm 46, with exemplary rays shown throughout. Light exits the fiberbundle (not shown) and enters the entrance face 51 of the prism 52. Theexiting face of the fiber bundle may be treated as an extended source,emitting rays into a cone of uniform size and direction, regardless oflocation in the bundle. This may be seen visually in FIG. 4 by carefulexamination of the ray bundles in close proximity to entrance face 51.The light then reflects off the hypotenuse 53 of the prism, andpropagates toward the exiting face 55 of the prism 52. The exiting face55 has a cylindrical element 57 with negative power that diverges thebeam in only one direction; in the other direction, the beam continuesto propagate with the divergence determined by the fiber bundlenumerical aperture. After leaving the prism 52, the beam reflects off ananamorphic reflector 54, which has different power in the x- andy-directions. The beam then strikes the illumination region 50 of thewafer 56 at a fairly high angle of incidence.

FIG. 6 shows a perspective view of the illumination system 40, with onlyone schematically rendered illumination arm 46 shown. The illuminationregion 50 on the wafer 56 is generally circular, as seen from thesurface normal 65. The same geometry of FIG. 6 is shown in a side viewin FIG. 7. The angle of incidence 64 can be quite large.

FIG. 8 shows a portion of the optical system along with some constructsthat are useful for defining the relevant geometry of the illuminationsystem. The wafer under test 61 has an illumination region 62, which isapproximately circular when viewed from roughly normal incidence. Normalincidence means that the observer or camera is located apart from thewafer 61 and has a viewing angle roughly parallel to a surface normal65. The illumination region 62 receives light from each of theilluminating arms, which may be located circumferentially around theillumination region 62. Any number of illuminating arms 46 may be used,including up to five or more.

A light cone 63 is shown converging to a particular location in theillumination region 62. This cone 63 does not represent the full extentof the illuminating beam, but represents all the light arriving at aparticular location from one illuminating arm. The cone 63 arrives witha particular angle of incidence 64, which can be substantial. In adark-field illuminator, the illumination is at a high enough incidentangle so that a specular reflection, which occurs on a flat portion ofthe wafer 61, reflects at a suitably high angle of reflection and is notcollected by the camera. In general, the higher the angle of incidencefor the illumination, the better. For a system in which the collectionoptics have a numerical aperture of 0.8, the minimum angle of incidencefor the dark field illumination is sin⁻¹ (0.8), or about 53°.Preferably, the angle of incidence is even higher than that, and may beas high as 81°, or higher.

The incident cone 63 has a particular plane of incidence, defined bydotted line 68 and surface normal 65. Strictly speaking, each ray in thecone has its own plane of incidence, but for the purposes of thisdocument, the plane of incidence for the entire cone 63 is defined asthe plane of incidence for the central ray in the cone 63.

The cone 63 is generally asymmetric, and has an elliptical angularprofile that may be defined by an incident angular range 66 and anazimuthal angular range 67, which is also referred to as acircumferential angle range. For all the rays in the cone 63, the trueangle of incidence at the wafer 61 is in the range of incident angle64±half the incident angular range 66. Note that the azimuthal angularcomponents do not affect the angle of incidence at the wafer 61. Ingeneral, it is desirable to minimize the incident angular range 66, sothat the illumination is more uniform. In discussions below, themeridional direction, which contains the incident angular range, isreferred to as the “x”-direction, while the azimuthal angular rangeextends along the “y”-direction.

Note that each location in the illumination region 62 has its own coneof incident rays, and that the cone 63 is drawn in FIG. 8 for only oneparticular location. It is important to note that the cone 63 hasessentially the same size for all locations within the field of view,meaning that the incident angle range 66 and azimuthal angle range 67are essentially invariant across the field of view. In other words, theangular spectrum is uniform across the field. This is a highly desirablefeature, and helps ensure that the sensitivity of the device isindependent of the location within the field of view.

It is instructive to attach some numerical values to these angularranges, which are obtained from the paraxial raytrace discussed below.These values are exemplary, and are presented to clarify the meaning ofsome of the quantities of FIG. 9.

In the x-direction, both the nominal angle of incidence 64 and theincident angle range 66 are truly invariant across the entire field ofview, or, equivalently, the entire illumination region 50. For everypoint inside the illumination region, the nominal angle of incidence 64is 81°. Likewise, for every point inside the illumination region, thefull incident angle range is 5°. In other words, for every point insidethe illumination region 50, the light arrives with an incident angle inthe range of 81°±2.5°.

In the y-direction, the azimuthal angle range 67 is invariant across thefield, but the nominal azimuthal angle does vary with location. At thecenter of the illumination region 50, the light arrives with anazimuthal angle in the range of 0°±0.4°. At one edge of the field, thelight arrives at 7.5°±0.4°, and at the other edge, it arrives at−7.5°±0.4°. The azimuthal angle range 67 is therefore 0.8° for allpoints in the illumination region 50.

The angular spectrum may be considered a combination of the incidentangle range 66 and the azimuthal angle range 67. From the abovediscussion, in which we showed that both of these quantities areinvariant for all point in the illumination region 50, we may also saythat the angular spectrum is invariant in the illumination region 50. Inaddition, the nominal incident angle 64 is invariant across the field.

The cone 63 drawn in FIG. 8 represents the output of one illuminationarm, for one particular inspection point in the illumination region 62.The full output beam from an illumination arm is a collection of cones63, with one for each location in the illumination region 62. The outputbeam is produced by an illumination arm described below, where thecomplete illumination system has multiple illumination arms, alllighting the same region 62 of the wafer 61, but from differentazimuthal directions. Much of the following description applies to asingle illumination arm, although it may apply to any or all the arms inthe complete system.

The elements from the fiber bundle to the wafer, or target, are ofspecial interest to us, in that the layout of these elements isnon-trivial. In order to address the specifics of these elements, we usea paraxial raytrace. FIG. 9 shows a schematic of an optical system thatis used for paraxial raytracing. From the raytrace, the required powersin x and y of the negative cylinder and the reflector are determined.

The multimode fiber bundle, shown at the leftmost edge of FIG. 9,radiates effectively uniformly into a rotationally symmetric cone with acharacteristic numerical aperture NA. For the preferred fiber bundle, NAis roughly 0.1519. If the fiber bundle were to radiate into air, theradiant cone would have a half-angle equal to sin⁻¹ NA. Because thefiber is coupled to the prism, so that the light leaving the fiberenters the prism, the radiant cone inside the prism has a half-angleequal to sin⁻¹ (NA/n_(glass)), where n_(glass) is the refractive indexof the prism. A high-index glass may be used for the prism, such asLaSFN9, which has a refractive index n_(glass) of 1.8558 at the designwavelength of 550 nm. Alternatively, other suitable glasses may be used,such as BK7 or other well-known glasses. Note that the required size ofthe fiber bundle is as yet undetermined.

The beam exiting the fiber bundle enters the glass prism, which firstbends the beam by 90 degrees, then diverges the beam in the x-dimensionby passing it through a cylindrical surface on the exiting face of theprism. The prism has a refractive index n_(glass), such as, for example,1.8558. The beam propagates an on-axis distance denoted by t_(glass),which includes the paths both before and after reflection from thehypotenuse of the prism. In order to satisfy the space requirement ofthe illumination system, a small prism is used, such as, for example,having a nominal square dimension of about 0.5 mm. Because of thecylindrical element on the exiting face of the prism, which extends intothe exiting face, the actual on-axis distance traveled by the beaminside the prism is less than 0.5 mm. The preferred thickness t_(glass)is about 0.474 mm, although other suitable values may be used fordifferent-dimensioned prisms.

The cylindrical surface on the exiting face of the prism has a surfacepower Φ_(1x); the corresponding power component along y, Φ_(1y), iszero. The beam exits into air, so the exiting refractive index n₁′ is 1.The curvature that yields a surface power of Φ_(1x) is found fromc_(1x)=Φ_(1x)/(n₁′−n₁), or c_(1x)=Φ_(1x)/(1−n_(glass)). The radius ofcurvature, R_(1x), is given by R_(1x)=1/c_(1x). There is no opticalpower along the y-dimension.

The beam exiting the negative cylindrical surface on the prism travelsthrough a distance t₁′ in air before entering the anamorphic reflector.Following the usual sign conventions used in ray tracing, the distancet₁′ may be denoted by t₂; both are numerically equal and areinterchangable. Both are also as yet undetermined. Because the beamtravels in air, the refractive index n₁′=n₂=1.

The anamorphic reflector has a reflective surface with different powersalong its x- and y-directions, denoted by Φ_(2x) and Φ_(2y). Thereflector may be a mirror having a highly reflective dielectric thinfilm stack, although other suitable types of reflectors may be used aswell. For this type of reflector, in which the incident and reflectivesurfaces are both air, the incident refractive index n₂ is 1, while theexiting refractive index n₂′ is −1. The mirror curvature that produces agiven power is given by c_(2x)=Φ_(2x)/(n₂′−n₂), or −Φ_(2x)/2. Similarly,c_(2y)=−Φ_(2y)/2. The radii of curvature are given by R_(2x)=1/c_(2x),and R_(2y)=1/c_(2y).

After exiting the reflector, the beam propagates a distance t₂′ to thetarget. Note that with the typical conventions of raytracing, both thevalues of distance t₂′ and refractive index n₂′ are negative afterreflection off the mirror; this is merely a numerical convenience, andsimply implies that the reflected beam travels in generally the oppositedirection as the incident beam. The distance t₂′ is as yet undetermined.

The target is the portion of the wafer that is to be inspected. It ispreferred that the incident beam illuminates a generally circular regionof the wafer. Because the incident beam strikes the target at a fairlyhigh angle of incidence I, the beam footprint exiting the system shouldbe asymmetric, so that a normally-incident camera may see a generallysymmetric illumination region. Consider the following geometry, wherethe plane of incidence includes the x-dimension, so that y-axis isparallel to the target surface. Along the y-dimension, the illuminationregion seen by the camera is the same size as the incident beam alongthe y-dimension, say 2F. Along the x-dimension, an incident beam of size(2F cos I) illuminates an region of dimension 2F as seen by anormally-incident camera. The incident beam is therefore smaller alongthe x-dimension by a factor of cos I, so that a normally-incident camerasees a circle of illumination with diameter 2F. For a large angle ofincidence, the compression factor cos I may be quite significant. Forexample, for the preferred angle of incidence of about 81°, cos (81°)equals 0.1564, resulting in a beam with an aspect ratio of about 6. Evenwith such a high aspect ratio, the correspondingly high angle ofincidence produces a nearly round beam, as seen by a camera at nearlynormal incidence. A preferred value for F is 10 mm, although othersuitable values may be used.

From the raytrace described below, values may be obtained for thecylindrical powers of the negative cylinder and the reflector, as wellas the distances between the surfaces.

Given the power Φ of each surface, the refractive index n between thesurfaces, and thickness t between the surfaces, one may use thewell-known paraxial refraction and transfer equations to trace a raythrough the optical system of FIG. 9.

The paraxial refraction equation predicts the exiting ray angle(relative to the optical axis) u′, after refraction at a surface withpower Φ:n′u′=nu−yΦ,where u is the incident ray angle, y is the incident and exiting rayheight at the surface, and n and n′ are the incident and exitingrefractive indices, respectively. The refractive indices aredimensionless, the ray angles are in radians, the ray heights are in mm,and the surface powers are in mm⁻¹.

The paraxial transfer equation predicts the ray height y′ at a surface,after propagation by a distance t between a previous surface and thecurrent surface:y′=y+tu,where y is the ray height at the previous surface and u is the ray angle(relative to the optical axis) between the previous surface and thecurrent surface. The ray angle is in radians and the ray heights anddistances are both in mm.

The above paraxial refraction and transfer equations are alternatelyused to trace rays through the multi-surface optical system of FIG. 8.Separate raytraces are performed for the x- and y-dimensions, with thethicknesses between the surfaces remaining the same, but the powers ofeach surface being different.

Some initial assumptions are made for the optical system. First, it isdesirable to image the pupil of the fiber bundle onto the target. For afiber bundle, the pupil of the bundle is located at infinity, so wetherefore locate the target at the rear focal plane of the system inboth x- and y-directions. Second, it desirable to illuminate a circularportion of the target. Because the illumination is at a fairly highangle of incidence, the incident beam should have an asymmetricfootprint. Therefore, the optical system should have differentmagnifications in the x- and y-dimensions, or, equivalently, differentfocal lengths along the x- and y-dimensions. We therefore choose toplace the fiber bundle at the front focal plane of the system in thex-direction, so that any spatial variations in intensity at the fiberbundle are minimized at the target. We do not explicitly constrain thefront focal plane of the system in the y-direction.

The goal of the ray trace, sketched schematically in FIG. 9, is toobtain values for cylindrical powers Φ_(1x), Φ_(2x) and Φ_(2y), anddistances t₂ and t₂′. The fixed values in the ray trace are therefractive index of the prism n_(glass) (equal to 1.8558, for example),thickness traveled inside the prism t_(glass) (equal to 0.474 mm, forexample), numerical aperture of the beam exiting the fiber bundle NA(equal to 0.1519, for example), and illuminated diameter at the target2F (equal to 20 mm, for example). Four rays are traced and are eachshown schematically, but not to scale, in FIG. 9.

First, we trace Ray 1 along the y-dimension. Omitting the intermediatealgebra, we find that the total power of the system in the y-dimensionΦ_(y) equals the power of the reflector in the y-dimension Φ_(2y), andboth are given by Φ_(y)=Φ_(2y)=NA/F. For the preferred values of NA(0.1519) and F (10 mm), Φ_(2y)=−0.01519 mm⁻¹, leading to a reflectorradius R_(2y) of −131.7 mm.

Next, we trace Ray 2 in the y-dimension. Again omitting the intermediatealgebra, we find that the on-axis distance between the reflector and thetarget t₂′ is given by t₂′=−F/NA. Using the preferred values above, t₂′is −65.8 mm. The value is negative, meaning that the incident andexiting beams are on the same side of the reflector; this is the usualcase for a mirror.

Next, we trace Ray 3 in the x-dimension. We obtain two pieces ofinformation from Ray 3. First, the total optical power of the system inthe x-direction is given by Φ_(x)=NA/(F cos I). For the preferredvalues, the total optical power in x is about +0.0971 mm⁻¹. Second, therelationship between thickness t₂ and power Φ_(2x) is given by(t₂)(Φ_(2x))=1−[(t_(glass)/n_(glass))(NA)/(F cos I)].

Finally, we trace Ray 4 in the x-dimension, and obtain a relationshipbetween thickness t₂ and power Φ_(1x), given by (t₂)(Φ_(1x))=1−(1/cosI).

We combine the preceding two equations with the well-known relationshipΦ_(x)=Φ_(1x)+Φ_(2x)−(t₂/n₂)(Φ_(1x))(Φ_(2x)) to obtain expressions forthe remaining powers and thickness.

The power along the x-direction for the negative cylinder is given by:Φ_(1x)=[1−(1/cos I)]/[(Fcos I/NA)−{(t _(glass) /n _(glass))/cos I)}].For the preferred values given above, Φ_(1x) is −0.622 mm⁻¹, giving aradius R_(1x) of +1.375 mm.

The power along the x-direction for the reflector is given by:Φ_(2x)=[1−(t _(glass) /n _(glass))(NA)/(F cos I)]/[(F cos I/NA)−{(t_(glass) /n _(glass))/cos I}]For the preferred values given above, Φ_(2x) is +0.1125 mm⁻¹, giving aradius R_(2x) of −17.77 mm.

The on-axis thickness t₂ (or, equivalently, t₁′) between the negativecylinder and the reflector target is given by:t ₂=(F cos I/NA)−[(t _(glass) /n _(glass))/cos I]For the preferred values given above, t₂ is +8.666 mm.

There is a remaining quantity that may be obtained from the trace of Ray4 in the x-direction. Because the fiber bundle is located at the frontfocal plane of the system (in x), all the rays that originate from aparticular point on the bundle arrive at the target at the same angle(in x). As a result, it becomes apparent that the size of the fiberbundle determines the range of incident angles (in x) that arrive at thetarget. If the bundle were infinitesimally small, all the rays wouldarrive at precisely the same incident angle, and the range of incidentangles would be essentially zero. Obviously, for radiometric reasons thebundle cannot be infinitesimally small; no light would get through thesystem. A reasonable value of the half-range of incident angles (in x)is roughly 2.5°, based on a compromise between radiometric power at thetarget (where a big bundle is suitable) and uniformity of incident angleat the target (where a small bundle is suitable). For a half-range HR ofincident angles, the required half-height of the bundle H is given byH=(tan HR)(F cos I)/NA. For the typical values given above, and ahalf-range HR of 2.5°, the required half-height of the fiber bundle isabout 0.45 mm. In other words, if the fiber bundle has a radius of 0.45mm, then all the illumination arrives at the target with an incidentangle of 81°±2.5°. For improved performance, the gap between theillumination angles and the collection angles may be made substantial,so that a specular reflection off the target remains well outside of thecollection optics.

Note that once the radii of curvature are determined for the varioussurfaces, aspheric and/or conic terms may be optionally added to one ormore of them to reduce aberrations in the beam. These aspheric terms aremost easily handled by a commercially available raytracing program, suchas Oslo, ZEMAX, Code V, and others.

Now that the values of powers, radii of curvature, and thickness havebeen determined in analytical form, based on the paraxial raytrace ofthe system shown in FIG. 9, it is instructive to show the numericalvalues of the corresponding ray traces. These values are given in theray trace spreadsheet shown in FIG. 10. The input values not determinedby the spreadsheet are shown in the thick-bordered cells.

A variety of rays are traced, including rays through the center and bothedges of the target, and through the center and one edge of the fiberbundle. Because the system is symmetric, a ray through the other edge ofthe fiber bundle is unnecessary. Note that rays 1 through 4 from FIG. 9are denoted in FIG. 10.

There are several noteworthy features of the spreadsheet of FIG. 10. Allthe rays originate at the fiber bundle with a height between −0.449 mmand +0.449 mm, and with a numerical aperture between −0.1519 and+0.1519. All the rays arrive at the target with a height between −10 mmand +10 mm in y, and between −1.564 mm and +1.564 mm in x. As discussedearlier, the beam is effectively expanded in x to appear the same sizeas in the y-dimension, with respect to a normally-incident camera.

In the x-dimension, all the rays arrive with an incident ray slopebetween −0.0437 radians and +0.0437 radians, or, equivalently, anincident ray angle between −2.5° and +2.5°. Because the x-dimension liesin the plane of incidence, the range of ±2.5° becomes the range ofincident angles at the target.

In contrast, note that in the y-dimension, the incident rays have a muchlarger angular range, with slopes between −0.138 radians and +0.138radians, or, equivalently, incident ray angles between −7.9° and +7.9°.Because the y-dimension is parallel to the target surface, this ratherlarge angular range has no effect on the range of incident angles,unlike the x-dimension. It is easiest to visualize this y-range from thepoint of view of a normally-incident camera, looking at the target. Ifone imagines a clock face superimposed on the target, with theillumination arm located near 12-o'clock, then portions of the 20mm-diameter circle at the center that receives illumination receive itfrom rays originating between roughly 11:45 and 12:15. In the actualvisual inspection station, five illumination arms are used, each spacedapart by about 72° around the full 360° clock-face envisioned earlier.It is understood that more or fewer than five illumination arms may beused as well.

It is instructive to summarize the paraxial layout thus far. A beamemerges from a multi-mode fiber bundle, passes through a 90° turningprism that has a negative-power cylindrical surface on its exiting face,reflects off an anamorphic mirror, and illuminates a target at anon-zero incident angle. The illumination region is generally round whenviewed by an essentially normally incident camera. The target is at therear focal plane of the system in both x- and y-dimensions. The fiberbundle is at the front focal plane of the system in the x-dimension. Thesize of the illumination region at the target is directly proportionalto the numerical aperture of the fiber bundle. The range of exitingangles from the illumination arm is directly proportional to the size,or spatial extent, of the fiber bundle.

Note that although the paraxial layout is performed as if all theelements are centered about the optical axis, in reality, they may bebent or folded to include non-normally-incident surfaces. For instance,the anamorphic mirror is treated validly as an on-axis element in theparaxial raytrace, but once the powers and distances are determined, theactual part may have a finite angle of incidence on-axis. These anglesare best determined by clear aperture requirements, where neither anincident nor a reflected beam should be inadvertently blocked by anycomponents. Because the incident beam is tilted with respect to thetarget, the center of the illumination region should be located at theprescribed z-distance, namely at the rear focal planes in both x and y.

The resulting system has many advantages over known illuminationsystems. For instance, the system is compact, has a large format (20 mmdiameter illumination region), has uniform illumination over theillumination region, has a uniform angular spectrum over theillumination region, has a small incident angle range, has a broadcircumferential azimuth (or, equivalently, a large azimuthal anglerange), and is relatively bright. As a result of these advantages, thewafer inspection system is essentially insensitive to particle locationand orientation within the field of view.

In all of the discussion thus far, it has been implicitly assumed thatthe optical power on the exiting face of the prism is essentiallycylindrical, with a negative power in the meridional direction and nopower in the azimuthal direction. As an alternative embodiment, powermay be provided in both dimensions on the exiting face of the prism, notjust in the meridional direction. In other words, the exiting face mayhave an element with anamorphic optical power, in which the powers inthe meridional and azimuthal directions are unequal; this may bereferred to as a first anamorphic element. Likewise, the anamorphicmirror or lens may be referred to as a second anamorphic element.

The optical path for this embodiment is similar to that of theessentially cylindrical embodiment and is shown in FIG. 11. Light exitsthe end of a multimode fiber bundle (not shown), enters a turning prism91 through its incident face 92, reflects off its hypotenuse (althoughother suitable geometries may be used), and exits the turning prism 91through its exiting face 93. The exiting face 93 has a first anamorphicelement 94 with optical power in both the meridional and azimuthaldimensions. The first anamorphic element 94 may be an anamorphicdepression in the surface of the exiting face 93, or may be ananamorphic lens attached to the exiting face. The beam exiting the prism91 propagates to a second anamorphic element 95, which may be a mirroror a lens. The beam exiting the second anamorphic element 95 thenstrikes the illumination region 96 at a high angle of incidence, withrespect to the surface normal 97.

For such an embodiment, it is still desirable to image the pupil of thefiber bundle onto the illumination region, which is ensured by placingboth the meridional rear focal plane and the azimuthal rear focal planeat the illumination region. In addition, because it is also desirable tohave a uniform nominal incident angle across the illumination region, weplace the fiber bundle at the meridional front focal plane. For thisembodiment, the location of the azimuthal front focal plane isindeterminate; it may or may not coincide with the meridional frontfocal plane.

As a further embodiment, a rotationally symmetric element may beprovided on or attached to the exiting face of the prism, rather than ananamorphic element or a purely cylindrical element.

The description of the invention and its applications as set forthherein is illustrative and is not intended to limit the scope of theinvention. Variations and modifications of the embodiments disclosedherein are possible, and practical alternatives to and equivalents ofthe various elements of the embodiments would be understood to those ofordinary skill in the art upon study of this patent document. These andother variations and modifications of the embodiments disclosed hereinmay be made without departing from the scope and spirit of theinvention, as set forth in the following claims.

1. An illuminator for dark field inspection of a surface of a target,comprising: one or more illumination arms disposed about an illuminationregion, each arm comprising: an extended, substantially uniform sourceof diverging light; a turning element having a first face opticallycoupled to the light source, and a second face having optical power in ameridional direction; and an anamorphic element having a first opticalpower in an azimuthal direction and a second optical power in themeridional direction, the first and second optical powers being unequal,and the anamorphic element being optically coupled to the second face ofthe turning element.
 2. The illuminator of claim 1, wherein the secondface of the turning element has no power in the azimuthal direction. 3.The illuminator of claim 1, wherein the second face of the turningelement has optical power in the azimuthal direction.
 4. The illuminatorof claim 1, wherein the anamorphic element comprises a mirror.
 5. Theilluminator of claim 1, wherein the anamorphic element comprises a lens.6. The illuminator of claim 1, wherein the turning element comprises: aturning prism; and a negative cylindrical lens attached to a face of theturning prism; wherein the first face of the turning element is anincident face of the turning prism, and the second face of the turningelement is an exiting face of the cylindrical lens.
 7. The illuminatorof claim 1, wherein: the turning element comprises a turning prismhaving an incident face and an exiting face, the exiting face containinga concave cylindrical surface; and the first face of the turning elementis the incident face of the turning prism, and the second face of theturning element is the exiting face of the turning prism.
 8. Theilluminator of claim 1, wherein each arm has an azimuthal front focalplane, an azimuthal rear focal plane, a meridional front focal plane,and a meridional rear focal plane, and wherein for each arm: theazimuthal front focal plane and the meridional front focal plane are notcoincident; the light source is located substantially at the meridionalfront focal plane; the meridional rear focal plane and the azimuthalrear focal plane are substantially coincident; the illumination regionis substantially circular with a center located substantially at themeridional rear focal plane and at the azimuthal rear focal plane; andthe light source, turning element, and anamorphic element are arrangedto illuminate the illumination region at a non-zero angle of incidencerelative to a normal of the illumination region to achieve specularreflection from the target surface under defect-free conditions.
 9. Themethod of claim 8, wherein the illumination region is within a field ofview of a collection lens having a numerical aperture that defines amaximum collection angle of light for the collection lens, the angle ofincidence being greater than the maximum collection angle.
 10. Theilluminator of claim 8, wherein the angle of incidence is greater thanapproximately 53 degrees.
 11. The illuminator of claim 10, wherein theangle of incidence is greater than approximately 81 degrees.
 12. Theilluminator of claim 1, wherein the light source operationally produceslight having a finite spectrum.
 13. The illuminator of claim 1, whereinthe light source comprises a high pass ultraviolet filter.
 14. Theilluminator of claim 1, wherein the cylindrical optical power isnegative.
 15. The illuminator of claim 1 wherein the illuminatorcomprises five arms.
 16. An illuminator for dark field inspection of asurface of a target, comprising: one or more illumination arms disposedabout a substantially circular illumination region substantially at thetarget surface, and each the arm comprising: a light source; a couplingelement having an incident face optically coupled to the light source,and an exiting face with negative cylindrical optical power in ameridional direction; and an anamorphic element having a first opticalpower in an azimuthal direction and a second optical power in themeridional direction, the first and second optical powers being unequal,and the anamorphic element being optically coupled to the exiting faceof the coupling element; wherein each of the arms has an azimuthal frontfocal plane, an azimuthal rear focal plane, a meridional front focalplane, and a meridional rear focal plane; and wherein for each of thearms: the azimuthal front focal plane and the meridional front focalplane are not coincident; the light source is located substantially atthe meridional front focal plane; the meridional rear focal plane andthe azimuthal rear focal plane are substantially coincident; theillumination region has a center located substantially at the meridionalrear focal plane and at the azimuthal rear focal plane; and the lightsource, turning element, and anamorphic element are arranged toilluminate the illumination region at a non-zero angle of incidencerelative to a normal of the illumination region to achieve specularreflection from the target surface under defect-free conditions.
 17. Themethod of claim 16, wherein the illumination region is within a field ofview of a collection lens having a numerical aperture that defines amaximum collection angle of light for the collection lens, the angle ofincidence being greater than the maximum collection angle.
 18. Theilluminator of claim 16, wherein the angle of incidence is greater thanapproximately 53 degrees.
 19. The illuminator of claim 16, wherein theangle of incidence is greater than approximately 81 degrees.
 20. Theilluminator of claim 16, wherein the light source operationally produceslight having a finite spectrum.
 21. The illuminator of claim 16, whereinthe light source comprises a high pass ultraviolet filter.
 22. Theilluminator of claim 16, wherein the anamorphic element comprises amirror.
 23. The illuminator of claim 16, wherein the anamorphic elementcomprises a lens.
 24. The illuminator of claim 16, wherein the couplingelement comprises a turning element.
 25. The illuminator of claim 16,wherein the coupling element comprises a non-turning element.
 26. Theilluminator of claim 16 wherein the light source is an extended,substantially uniform source of diverging light.
 27. The illuminator ofclaim 16 comprising five illumination arms.
 28. An illuminator for darkfield inspection of a surface of a target, comprising: a plurality ofillumination arms disposed about an illumination region substantially atthe target surface, and each of the arms comprising: an extended,substantially uniform source of diverging light; and an assembly ofoptical elements for producing from the diverging light an illuminatingbeam directed to the illumination region, the illuminating beam having anominal incident angle, an incident angle range, and an azimuthal anglerange; wherein both the nominal incident angle and incident angle rangeare essentially invariant throughout the illumination region; whereinthe azimuthal angle range is essentially invariant throughout theillumination region; and wherein light intensity throughout theillumination region is essentially invariant.
 29. An illuminator fordark field inspection of a surface of a target, comprising: a pluralityof illumination arms disposed about a substantially circularillumination region substantially at the target surface, and each of thearms comprising: means for establishing an azimuthal rear focal plane;means for establishing a meridional rear focal plane, the meridionalrear focal plane and the azimuthal rear focal plane being substantiallycoincident; means for establishing an azimuthal front focal plane; meansfor establishing a meridional front focal plane separated from theazimuthal front focal plane; means for locating a center of theillumination region substantially at the meridional rear focal plane andat the azimuthal rear focal plane; means for introducing light havingextended, substantially uniform, and diverging characteristicssubstantially at the meridional front focal plane; and means for forminga beam from the light introduced at the meridional front focal plane,the beam being incident on the illumination region at a non-zero angleof incidence relative to a normal of the illumination region to achievespecular reflection from the target surface under defect-freeconditions.
 30. A method for illuminating an illumination region on asurface of a target from a plurality of different directions withrespective optical systems to perform a dark field inspection of thetarget surface, comprising for each of the optical systems: establishingan azimuthal rear focal plane; establishing a meridional rear focalplane, the meridional rear focal plane and the azimuthal rear focalplane being substantially coincident; establishing an azimuthal frontfocal plane; establishing a meridional front focal plane separated fromthe azimuthal front focal plane; establishing the illumination region asa substantially circular region with a center substantially at themeridional rear focal plane and at the azimuthal rear focal plane;introducing light having extended, substantially uniform, and divergingcharacteristics substantially at the meridional front focal plane; andforming a beam from the light introduced at the meridional front focalplane, the beam being incident on the illumination region at a non-zeroangle of incidence relative to a normal of the illumination region toachieve specular reflection from the target surface under defect-freeconditions.
 31. The method of claim 30, wherein the beam forming stepcomprises forming the beam within a field of view of a collection lenshaving a numerical aperture that defines a maximum collection angle oflight for the collection lens, the angle of incidence being greater thanthe maximum collection angle.
 32. The method of claim 30, wherein theangle of incidence in the beam forming step is greater than about 53degrees.
 33. The method of claim 32, wherein the angle of incidence inthe beam forming step is greater than about 81 degrees.
 34. The methodof claim 30, wherein the beam forming step comprises spectrallyfiltering the beam.
 35. A wafer inspection system comprising: a fiberoptic dark-field illuminator comprising: a source of spectrallyfiltered, spatially uniform, extended and diverging light; and aplurality of illumination arms optically coupled to the light source andcircumferentially disposed about a substantially circular illuminationregion; and a camera disposed at normal or near-normal incidence withrespect to the illumination region, the illumination region being withina field of view of the camera, and the camera comprising a collectionlens having a numerical aperture that defines a maximum collection angleof light for the collection lens; wherein each of the illumination armscomprises: a turning element having an incident face optically coupledto the light source, and an exiting face with negative cylindricaloptical power in a meridional direction; and an anamorphic mirror havinga first optical power in an azimuthal direction and a second opticalpower in the meridional direction, the first and second optical powersbeing unequal, and the anamorphic mirror being optically coupled to theexiting face of the turning prism; wherein each of the illumination armshas an azimuthal front focal plane, an azimuthal rear focal plane, ameridional front focal plane, and a meridional rear focal plane; andwherein for each of the arms: the azimuthal front focal plane and themeridional front focal plane are not coincident; the light sourcecoupling is located substantially at the meridional front focal plane;the meridional rear focal plane and the azimuthal rear focal plane aresubstantially coincident; the illumination region has a center locatedsubstantially at the meridional rear focal plane and at the azimuthalrear focal plane; and the light source coupling, turning element, andanamorphic mirror are arranged to illuminate the illumination region atan angle of incidence relative to a normal of the illumination regionthat is greater than the maximum collection angle.
 36. The method ofclaim 35, wherein the turning element comprises a turning prism.
 37. Anilluminator for dark field inspection of a surface of a target,comprising: a plurality of illumination arms disposed about anillumination region, each arm comprising: a light source; a couplingelement having a first face optically coupled to the light source, and asecond face having optical power in a meridional direction; and ananamorphic element having a first optical power in an azimuthaldirection and a second optical power in the meridional direction, thefirst and second optical powers being unequal, and the anamorphicelement being optically coupled to the second face of the turningelement.
 38. The illuminator of claim 37, wherein the second face of theturning element has no power in the azimuthal direction.
 39. Theilluminator of claim 37, wherein the second face of the turning elementhas optical power in the azimuthal direction.